1. Field of the Invention
The invention relates to a method for performing Plasma Chemical Vapor Deposition (PCVD), whereby one or more layers of silica can be deposited on an elongated vitreous substrate.
2. Description of the Related Art
The term “silica” should here be interpreted as referring to any substance of the form SiOx, whether stoichiometric or not, and whether crystalline or amorphous.
Such an apparatus is well known in the art of optical fiber manufacture, for example, and can be used in that context to manufacture a so-called preform rod from which an optical fiber can be drawn. In one known method of manufacturing such a preform rod, a straight vitreous substrate tube (comprised of quartz, for example) is coated on its inside cylindrical surface with layers of doped silica (e.g., germanium-doped silica). This can be achieved by positioning the substrate tube along the cylindrical axis of the resonant cavity, and flushing the example); a localized plasma is concurrently generated within the cavity, causing the reaction of Si, O and Ge so as to produce direct deposition of Ge-doped SiOx on the inside surface of the substrate tube. Since such deposition only occurs in the vicinity of the localized plasma, the resonant cavity (and thus the plasma) must be swept along the cylindrical axis of the tube in order to uniformly coat its whole length. When coating is completed, the tube is thermally collapsed into a rod having a Ge-doped silica core portion and a surrounding undoped silica cladding portion. Typically, such a rod is of the order of about 1 m long and 2 cm wide. If an extremity of the rod is heated so that it becomes molten, a thin glass fiber (typically of the order of about 125 μm wide) can be drawn from the rod and wound on a reel; this fiber then has a core and cladding portion corresponding to those of the rod. Because the Ge-doped core has a higher refractive index than the undoped cladding, the fiber can act as a waveguide, e.g., for use in propagating optical telecommunications signals. It should be noted that the gaseous mixture flushed through the substrate tube may also contain other components; e.g., the addition of C2F6 causes a reduction in the refractive index of the doped silica. It should also be noted that the preform rod may be placed in a so-called jacket tube (comprised of undoped silica) prior to the drawing procedure, so as to increase the quantity of undoped silica relative to doped silica in the final fiber.
The use of such a fiber for telecommunications purposes requires the fiber to be substantially free of flaws (e.g., discrepancies in dopant concentration, unwanted cross-section ellipticity, etc.) since, when considered over great lengths of the fiber, such flaws can cause serious attenuation of the carried signal. As a result, it is important that the PCVD process be highly uniform, since the quality of the deposited PCVD layers will ultimately determine the quality of the fiber; accordingly, it is important that the plasma generated in the resonant cavity be highly rotationally symmetric (about the cavity's cylindrical axis). On the other hand, the economy of the production process would be greatly improved if the preform rod could be made thicker, since greater lengths of fiber could then be obtained from a single rod. However, these two goals are difficult to unify, since increasing the diameter of the resonant cavity so as to allow use of a thicker substrate tube generally leads to a plasma with deteriorated rotational symmetry; moreover, such a plasma can only be generated at the expense of a much higher microwave power.